One of the laws of physics says that planets and stars will keep on spinning unless a certain kind of force acts on them. This same law helps keep your bicycle upright while you’re riding it.
When you ride a bicycle slowly, it’s mostly you that keeps the bike standing up. Your ability to balance helps keep the bike from falling over. As you ride, you lean ever so slightly to the left and right in order to hold steady.
But the laws of physics help keep the bike from falling, too — especially when you’re going faster. That’s because spinning wheels have a special property called “angular momentum.” It’s one of the basic laws of physics that angular momentum has to be conserved, or stay the same. What this means is that a spinning wheel will just keep on spinning — in the same direction — unless it’s acted on by a certain kind of force.
Falling over would mean a change in direction — and the wheels resist this change. One way to demonstrate this to yourself is to take a wheel off your bike, and hold the wheel upright. If someone spins the wheel and then you try to turn it on its side, you might get the wheel to twist a bit. But you’d have to be amazingly strong to tip it over sideways.
In much the same way, when you ride a bike, the spinning of the wheels helps keep the bike upright.